The present invention relates to a driving apparatus for a thermal head, or more in particular to a thermal head driving apparatus having a heat storage compensation circuit suitable for a high-speed, high-quality thermal recording.
The thermal head used for thermal recording comprises a plurality of heat-generating elements in alignment. Only the required ones of the heat-generating elements are heated in correspondence with image data to generate a color on thermal recording paper, and ink on an ink film is transferred onto the recording paper for recording.
In a recording operation using such a thermal head, an increase in recording speed causes the printing of the next line before sufficient diffusion and discharge of thermal energy applied to the heat-generating elements, and thermal energy is thus stored steadily in the heat-generating elements. As a result, it is conventionally experienced that each heat-generating element stores therein thermal energy corresponding to the heat generation history thereof, thereby leading to variations in energy state to cause a deteriorated image quality.
In the transfer of ink from an ink film to the recording paper, for example, a method of applying the same thermal energy to each heat-generating element without giving due consideration to the heat-generation history thereof would accumulatively add the heat storage energy and the resulting total energy would lead to an increased amount of transferred ink, thereby blurring the printed characters or making it impossible to secure the desired area gradation by tone production method by density of each element.
In order to obviate the deterioration in image quality, a heat storage compensation method has been already proposed in which a proper quantity of energy is computed for application to an objective heat-generating element from the current recording performance of the objective heat-generating element and the recording history of adjacent heat-generating elements. The method, however, requires storing of the recording history of each heat-generating element and requires, for accurate compensation, referring to a wide range of recording data, accordingly requiring a great capacity of memory.
Several attempts have been hitherto made to develop a method of more accurate heat storage compensation free of a large capacity memory which is a disadvantage of the above-mentioned method.
The disclosure of JP-A-60-161163, for example, shows a heat storage compensation system for calculating the heat storage condition of each heat-generating element for the next occasion of recording from a current heat storage condition of each heat-generating element and an energy currently applied thereto, and compensating the energy applied to the respective heat-generating element on the next occasion on the basis of the calculated storage condition.
In the above heat storage compensation system, the difference between a target energy and the energy condition of each heat-generating element stored in an energy-condition buffer is used as an energy to be applied to the particular heat-generating element. Further, an applied energy computation circuit compensates for the effect of mutual thermal reaction between any objective or current heat-generating element and peripheral heat-generating elements thereby to determine the optimum applied energy.
Further the value representing the energy to be applied to each heat-generating element determined by the applied energy computation circuit is added to a value representing the energy condition of the respective heat-generating element after one-line recording cycle calculated by a thermal diffusion computation circuit, and the addition is stored in an energy condition buffer as an energy condition of the respective heat-generating element for the next-line recording. The thermal diffusion computation circuit computes the thermal diffusion from the current energy conditions of each heat-generating element and peripheral heat-generating elements and from the temperature of the thermal head substrate and thus determines the energy condition after one-line recording cycle.
The computation process at the thermal diffusion computation circuit of such a conventional heat storage compensation system is so complex that it is difficult to meet the requirements of high recording speed and high definition.
In the case of recording an original B4 in size at the rate of 400 DPI (dots/inch) and 2 msec per line, for instance, a total of 4096 heat-generating elements is required and it is necessary to compute the thermal diffusion of each heat-generating element within 500.
This problem might be solved by use of a high-speed device or high-speed technique such as parallel processing or pipeline processing. The resulting increase in size and cost, however, makes the apparatus less practicable.